In the electronics field, conventionally, there is a wide use of polyimide resins having various excellent properties, such as high heat resistance, high electric insulating property, and the like. For example, such polyimide resins are used in flexible printed circuit boards, polyimide films for use in semiconductor packages such as, TAB tapes, base films for high-density recording media, laminates having a metal layer and a polyimide film by using PVD methods, and the like. Other than as films, the polyimide resins are used in various forms, such as molded solid forms, coating agents, and the like. In case of the polyimide films, the polyimide films have been used not only solely, but also as a laminate. Such laminates are prepared by: (a) bonding copper foil on a polyimide film via an adhesive agent; (b) metallizing copper directly on the polyimide film by using a sputtering method or the like and then performing electrolytic plating of copper thereon, or (c) casting the polyimide resin on copper foil, or coating the copper foil with the polyimide.
For example, when the polyimide films are used for flexible printed circuit boards, polyimide films for use in semiconductor packages such as COFs, TAB tapes, base films for high-density recording media, laminate having a metal layer and a polyimide film, and the like, the polyimide films should be prepared with high dimensional stability in order to attain necessary properties for the use. For the flexible printed circuit boards, TAB tapes and the like, the polyimide film that has been expanded in heating is bonded with the metal layer via the adhesive agent while still being expanded. For laminates having a metal layer and a polyimide film by using PVD (Physical Vapor Deposition methods, metal is vacuum deposited on a surface of the polyimide film by an evaporative deposition method or a sputtering method. In those cases, therefore, thermal shrinkage is caused after cooling the thus prepared laminate having a metal layer and the polyimide film, thereby causing dimensional changes of the laminate, or residual stress in adhering the metal and the polyimide film causes dimensional changes of the laminate in etching the copper foil. Such dimensional changes lead to misalignment between a copper foil pattern and a circuit pattern. The copper foil pattern being to be associated with an electric/electronic component when the electric/electronic component is mounted after circuit formation and mounting of an IC (integrated circuit) or an LSI (Large Scale Integration), and the circuit pattern being to be associated with a flexible wiring substrate.
In mounting the IC and LSI, the polyimide film on which a metal wiring is formed is soaked, together with the IC and LSI, into a solder bath of a high temperature (about 300° C.) (reflow-soldering step). When the polyimide film is exposed to a high temperature as such, misalignment is caused between the circuit pattern and the wiring pattern of the electric/electronic components (wiring section of the IC and LSI). In response to recent increase in environmental consciousness among companies, use of lead-free solder, which contains no lead and has a high melting point, has been increased, whereby thermal shrinkage property of polyimide film at high temperatures has become a great concern. Because of this, as a target of thermal shrinkage at a high temperature, it is desired that the polyimide has a low thermal shrinkage rate at a temperature of 300° C., for example.
Further, when the polyimide film is exposed to a high temperature in the reflow soldering process, hygroscopic moisture inside the polyimide film is evaporated off from the inside of the polyimide film, the evaporation of the moisture causes such problem that in a polyimide film on which metal is laminated, and the like, foam is produced on that surface of the polyimide film, which faces the metal, a polyimide film having a low coefficient of hygroscopic expansion, and a small water absorption, has been sought for prevention of this problem.
For example, Japanese Publication of Unexamined Patent Application, Tokukai, No. 2001-270034 (published on Oct. 2, 2001) (Publication 1) discloses a thermo compression bonding polyimide film whose thermal shrinkage rate at 300° C. is 0.1% or less. Publication 1 reads that it is possible to use a highly heat tolerant aromatic polyimide film made of pyromellitic acid, paraphenylenediamne, and 4,4′-diaminodiphenyl ether. However, the art described in Publication 1 requires that a thermoplastic resin be laminated on a surface of the polyimide film. However, it is difficult to perform the laminating in a general manufacturing process of polyimide film. Thus, a special manufacturing apparatus for performing the laminating is necessary. Further, in the polyimide film, on the surface of which the thermoplastic resin is laminated, it is a problem that the thermoplastic resin has a high water absorption percentage and a high coefficient of hygroscopic expansion. Further, it is also a problem that the thermoplastic resin is poorly heat tolerant (heat susceptible). Moreover, there has been no art that found and discuss a correlation of thermal shrinkage with (a) a temperature of the film (film temperature) when the film is at tan δ peak, and (b) a tan δ value of the film at 300° C., in order to have a small thermal shrinkage at 300° C.
Moreover, the recent improvement of electronic raw materials and apparatuses requires that the polyimide films for use therein have not only the basic properties such as heat resistance, inductiveness, solvent resistance, but also more complicate various properties.
One of such more complicate various properties is high stability against harsh environments. A main body of the polyimide film should be highly stable against harsh environments (harsh environment), because the polyimide film may be used in a harsh environment. For example, when used in a circuit substrate inside an automobile, or used for covering wiring inside an automobile, the polyimide film is exposed to a high-temperature and highly moist environment. When used for covering wring inside an aero plane, the polyimide film is exposed to an environment in which temperature is largely changed. Here, the stability of the main body of the film against harsh environments refers to, for example, stability against changes (both increase and decrease) in temperatures, stability against a harsh environment such as a high-temperature and highly moist condition, and the like.
Meanwhile, the metal laminated wiring board having a fine pattern, such as a flexible wiring board, is recently so processed that not only the surface of the polyimide film is fabricated so as to form a metal wiring layer thereon, but also the polyimide film itself is finely fabricated. For this reason, it is recently required that the polyimide film itself be suitable for such fine fabrication.
There are various methods of finely fabricating the polyimide film. Examples of such methods of finely fabricating the polyimide film are: stamping (blanking) process; plasma etching process; laser cutting process, alkali etching process, and the like. The alkali etching process, in which the polyimide film is processed with an alkali etching solution, draws an attention because the alkali etching process has low cost, needs only simple facility, and fabrication thereby is so easy.
Conventionally, most of such polyimide films have slow dissolution rate with respect to the alkali etching solution. Thus, there have been studies on use of an alkali etching solution that increases the dissolution rate of the polyimide film in order to improve efficiency of the etching of the polyimide film. Further, for example there have been studied on use of a special alkali etching solution that improves the alkali etching rate. (For example, Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 5-202206 (published on Aug. 10, 1993) (Publication 2), Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 10-97081 (published on Apr. 14, 1998) (Publication 3)). However, no study has been conducted on the polyimide film itself in order to improve efficiency of the etching of the polyimide film. Especially, no study has been carried out, for example, on a polyimide film having an improved alkali etching rate for an alkali etching solution having a low alkali concentration, such as a 1N potassium hydroxide.
On the other hand, Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 5-78503 (published on Mar. 30, 1993) (Publication 4) discloses a polyimide film using biphenyl tetracarboxylic dianhydride and pyromellitic dianhydride. Although Publication 4 notes alkali etching property of this polyimide film, the alkali-etching property thereof is not sufficient. More specifically, an alkali etching rate of this polyimide film is slow with respect to a low-alkali etching solution (having low alkali concentration). Therefore, it is necessary to select an alkali etching solution that is suitable for the polyimide.
Moreover, the recent improvement in the electronic raw materials and electronic apparatuses requires that the polyimide film for use therein be so tolerable against heat and tension that its dimension will not be changed much due to heat and tension. In short, as the electric/electronic apparatuses are downsized, the flexible printed circuit board for use therein should have finer wiring patters accordingly. As a result, polyimide film having no large dimensional change is required. The lower the coefficient of linear expansion, the smaller the heat-causing dimensional change. And, the higher the modulus of elasticity, the smaller the tension-causing dimensional change.
Generally speaking, in order to produce a polyimide film having a high modulus of elasticity and a low coefficient of linear expansion, a monomer containing pyromellitic dianhydride, paraphenylene diamine, or the like is used, for example. Such monomers have high rigidity and linearity. However, a polyimide film prepared from such monomers is so poor in flexibility and a flexible printed circuit board using the polyimide film is not flexible, even though the flexibility is an essential property of the flexible printed circuit board Moreover, besides the poor flexibility, such film has a high water absorption percentage and a high coefficient of hygroscopic expansion. In addition, for use in a semiconductor package, for example, the polyimide film should have a low water absorption percentage and a low coefficient of hygroscopic expansion, besides the properties discussed above.
Furthermore, recently wiring patterns on the polyimide films for use in the flexible printed circuit boards is so fabricated as to be finer. This requires a polyimide film to have such an arrangement that a thin metal film, on which such a fine pattern can be formed, is laminated thereon. Conventionally, the most popular method is to laminate a thin copper foil on a surface of polyimide film via an adhesive agent such as a thermoplastic polyimide-type adhesive agent or an epoxy-type adhesive agent, and the like. However, according to this method, it is difficult to laminate, on the surface of the polyimide film, a thin copper film that is suitable for forming the fine pattern thereon. Such lamination is necessary to satisfy the above-mentioned requirement for forming the fine patterns.
In view of this, a method of manufacturing a laminate having a metal layer and the polyimide film is getting popular recently. In this method, a thin metal film is formed on a surface of the polyimide film by using a sputtering apparatus or a metal vapor depositing apparatus. Then, on top of the thin metal film, copper is plated by using gold as a catalyst, so that copper is laminated thereon. By adopting this method, it is possible to have a metal layer of an arbitrary thickness such as a thickness of less than 1 μm, and a thickness of more than several ten μm. Especially, with this method, it is possible to manufacture a metal layer having an optimum thickness for forming the fine pattern, because a thin laminated film can be realized according to this method.
However, in case where such method of performing vapor deposition or sputtering vapor deposition of metal is adopted, (a) a peel strength at an interface between the metal and the metal layer formed on the polyimide film and (b) a peel strength at an interface between the metal layer and the polyimide film are varied depending on which type of the polyimide film is used or which composition the polyimide film has. In general, therefore, the surface of the polyimide film is modified, as pretreatment prior to the vapor deposition and sputtering of the metal. Examples of the pretreatment are an NaOH treatment, a plasma treatment under vacuum, a plasma treatment under normal pressure, a corona treatment under normal pressure, a sand blast treatment, and the like. However, there is such a drawback that such pretreatment requires a large-scaled apparatus. Further, even though such surface modification is very effective for improving initial peel strength, such surface modification damages the surface of the polyimide film, whereby, the peel strength cannot be maintained stably for a long period. Therefore, there is a strong demand for a polyimide film capable of giving a higher peel strength at an interface of metal and itself, the peel strength maintained even after the surface modification and stably maintained for a long period.
A number of studies have been conducted intensively, in order to obtain a polyimide film having a high modulus of elasticity, a low coefficient of linear expansion, a low water absorption percentage, and a low coefficient of hygroscopic expansion. In order to obtain such polyimide film, for example, it has been studied to produce a polyimide film from a long-chained monomer so that the polyimide film contains a less number of imide groups in its molecular structure.
Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 11-54862 (published on Feb. 26, 1999) (Publication 5) and Japanese Publication of Unexamined Patent Application, Tokukai, No. 2001-72781 (published on Mar. 21, 2001) (Publication 6) disclose polyimide films produced from a p-phenylene bis(trimellitic monoester anhydride). The polyimide films have a low water absorption percentage and a low coefficient of hygroscopic expansion. However, the polyimide films described in Publications 5 and 6 show poor stability in harsh environment resistance tests. Especially, it is a problem that metal adhesive strengths of the polyimide films cannot be retained (poor retention rate of the metal adhesive strength), in case metal is directly laminated on the polyimide films.
Further, Publication 4 and Publication 7, namely, Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 9-235373 (published on Sep. 9, 1997), disclose polyimide films prepared from biphenyl tetracarboxylic dianhydride, pyromellitic dianhydride, p-phenylenediamine, and 4,4′-diaminodiphenyl ether. However, Publications 4 and 7, which discuss about the water absorption percentage, lack description on coefficient of hygroscopic expansion. Publications 4 and 7 face such problems that their polyimide films have a high coefficient of linear expansion at a high temperature.
As discussed thus far, there has been no polyimide film satisfactorily having a low coefficient of hygroscopic expansion, a low coefficient of linear expansion, a high modulus of elasticity, a good metal adhesiveness (property to be adhered with metal), and a good metal adhesion after harsh environment resistance test (property to retain the property to be adhered with metal, even after exposed to a harsh environment).